Nd—Fe—B system permanent magnets have a growing range of application due to their excellent magnetic properties. Also in the field of rotary machines including motors and power generators, permanent magnet rotary machines utilizing Nd—Fe—B system permanent magnets were developed to meet the recent demand for size, profile and weight reductions, performance enhancement and energy saving. Permanent magnets are situated in rotary machines such that they are exposed to high temperature due to the heat generated by windings and cores and have a likelihood of demagnetization by the diamagnetic field from the windings. There thus exists a demand for Nd—Fe—B system sintered magnets in which the coercive force which is an index of heat resistance and demagnetization resistance is above a certain level and the remanence (or residual magnetic flux density) which is an index of the magnitude of magnetic force is as high as possible.
An increase in the remanence (or residual magnetic flux density) of Nd—Fe—B sintered magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made on the process. With respect to the increased coercive force, among different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of effective elements, the currently most common approach is to use alloy compositions having Dy or Tb substituent for part of Nd. Substituting these elements for Nd in the Nd2Fe14B compound increases both the anisotropic magnetic field and the coercive force of the compound. The substitution with Dy or Tb, on the other hand, reduces the saturation magnetic polarization of the compound. Therefore, as long as the above approach is taken to increase coercive force, a loss of remanence is unavoidable.
In Nd—Fe—B sintered magnets, the coercive force is given by the magnitude of an external magnetic field created by nuclei of reverse magnetic domains at grain boundaries. Formation of nuclei of reverse magnetic domains is largely dictated by the structure of the grain boundary in such a manner that any disorder of grain structure in proximity to the boundary invites a disturbance of magnetic structure, helping formation of reverse magnetic domains. It is generally believed that a magnetic structure extending from the grain boundary to a depth of about 5 nm contributes to an increase of coercive force (See K. D. Durst and H. Kronmuller, “THE COERCIVE FIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism and Magnetic Materials, 68 (1987), 63-75). The inventors found that by concentrating trace Dy or Tb only in proximity to the grain boundaries to increase the anisotropic magnetic field only in proximity to the boundaries, the coercive force can be increased while suppressing any decline of remanence (as disclosed in JP-B 5-31807). Subsequently, the inventors established a production method comprising separately preparing a Nd2Fe14B compound composition alloy and a Dy or Tb-rich alloy, mixing them and sintering the mixture (as disclosed in JP-A 5-21218). In this method, the Dy or Tb-rich alloy becomes a liquid phase during the sintering and is distributed so as to surround the Nd2Fe14B compound. As a consequence, substitution of Dy or Tb for Nd occurs only in proximity to grain boundaries in the compound, so that the coercive force can be effectively increased while suppressing any decline of remanence.
However, since the two types of alloy fine powders in the mixed state are sintered at a temperature as high as 1,000 to 1,100° C., the above-described method has a likelihood that Dy or Tb diffuses not only to the boundaries, but also into the interior of Nd2Fe14B grains. An observation of the structure of an actually produced magnet shows that Dy or Tb has diffused to a depth of about 1 to 2 μm from the boundary in a grain boundary surface layer, the diffused area reaching 60% or more when calculated as volume fraction. As the distance of diffusion into grains becomes longer, the concentration of Dy or Tb near the boundaries becomes lower. To positively suppress the excessive diffusion into grains, lowering the sintering temperature may be effective. However, this measure cannot be practically acceptable because it compromises densification by sintering. An alternative method of sintering at lower temperatures while applying stresses by means of a hot press or the like enables densification, but poses the problem of extremely reduced productivity.
On the other hand, it is reported that coercive force can be increased by machining a sintered magnet to a small size, applying Dy or Tb on the magnet surface by sputtering, and heat treating the magnet at a temperature lower than the sintering temperature, thereby causing Dy or Tb to diffuse only to grain boundaries (see K. T. Park, K. Hiraga and M. Sagawa, “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the Sixteen International Workshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257 (2000); and K. Machida, H. Kawasaki, M. Ito and T. Horikawa, “Grain Boundary Tailoring of Nd—Fe—B Sintered Magnets and Their Magnetic Properties,” Proceedings of the 2004 Spring Meeting of the Powder & Powder Metallurgy Society, p. 202). This method allows for more effective concentration of Dy or Tb at the grain boundary and succeeds in increasing the coercive force without a substantial loss of remanence. As the magnet becomes larger in specific surface area, that is, the magnet body becomes smaller, the amount of Dy or Tb fed becomes larger, indicating that this method is applicable to only compact or thin magnets. However, there is still left the problem of poor productivity associated with the deposition of metal coating by sputtering or the like.
Nd—Fe—B system sintered magnets are now used in rotary machines with a high capacity of 10 kW or greater. Nd—Fe—B system sintered magnets are conductors having an electric resistance of 100 to 200 μΩ-cm. The eddy current generated within a magnet and heat generation associated therewith increase in proportion to the square of the magnet size. This gives rise to a problem in high capacity rotary machines. Effective means for reducing eddy currents is insulated lamination of thin plates like magnetic steel sheets in cores. The method including bonding finely divided magnet segments and consolidating into a magnet of the desired size has the problem that more steps are involved in the magnet manufacturing process, leading to an increase of manufacturing cost and a lowering of magnet weight-basis yield. Although one might consider to use magnet segments as such without bonding and consolidation, it is difficult to assemble and secure discrete magnet segments in a rotor against the repulsion between the magnet segments.